1. Field of the Invention
The present invention relates to a coating composition for an electrophotographic photoreceptor that is applied to form a charge-generating layer constituting an electrophotographic photoreceptor, an electrophotographic photoreceptor produced by using the coating composition, a method for producing the electrophotographic photoreceptor, and an image-forming apparatus using the electrophotographic photoreceptor.
2. Description of the Related Art
Conventionally, for electrophotographic photoreceptors used in electrophotographic apparatuses to which Carlson process is applied, inorganic photoconductive materials such as a selenium alloy, zinc oxide, and cadmium sulfide have been used. Inorganic photoconductive materials have disadvantages such as being harmful for the human body, having poor formability, being heavy, and being expensive. In recent years, there has been active development of electrophotographic photoreceptors using organic photoconductive materials that are superior to inorganic photoconductive materials in terms of, for example, non-toxicity, formability, light weight, and low price.
For example, a bisazo compound is an organic photoconductive material. A bisazo compound generally has a good sensitivity to light in the short-wavelength region or the middle-wavelength region, but the sensitivity to light in the long-wavelength region is low. Therefore, a bisazo compound is difficult to put into practical use when a semiconductor laser is used as a light source. As organic photoconductive materials having a relatively good sensitivity to light in the long-wavelength region, for example, a squaric acid methine dye, an indoline dye, a cyanine dye, and a pyrylium dye are known. However, these materials do not have a sufficient stability in repeated use and are difficult to put into practical use. As organic photoconductive materials having a good sensitivity to light in the long-wavelength region and a relatively good stability in repeated use, a phthalocyanine compound is known and has been actively researched as an organic photoconductive material in recent years.
As organic electrophotographic photoreceptors using an organic photoconductive material, electrophotographic photoreceptors of a so-called function-separated and layered type are known in which its photosensitive layer is divided into a charge-generating layer for generating charged carriers by receiving light and a charge-transporting layer for transporting the generated charged carriers. In these organic electrophotographic photoreceptors of the function-separated and layered type, each of the layers is formed of an optimal material for exerting its function, and these layers are combined, so that the sensitivity can be improved significantly and the sensitivity to the wavelength of light for exposure can be accordingly enhanced. Because of these advantages, electrophotographic photoreceptors of the function-separated and layered type have come to the mainstream of development, are being put into practical use, and are used in electrophotographic apparatuses such as copiers, printers, and facsimiles. In particular, electrophotographic photoreceptors formed by using oxotitanylphthalocyanine, which is a photoconductive material, as a charge-generating substance are often used in digital image-forming apparatuses because of their high sensitivity to light in the long-wavelength region, but there is the problem that the cost is relatively high.
An organic electrophotographic photoreceptor of the function-separated and layered type has a photosensitive layer including a charge-generating layer and a charge-transporting layer. The photosensitive layer is produced as follows; a coating composition for an electrophotographic photoreceptor in which an organic charge-generating substance and a binding resin are dispersed or dissolved in an organic solvent is applied onto a hollow cylindrical conductive substrate and dried to form a charge-generating layer; then a coating composition for an electrophotographic photoreceptor in which a charge-transporting substance and a binding resin are dispersed or dissolved in an organic solvent is applied thereon and dried to form a charge-transporting layer.
A photosensitive layer is required to be a thin film and to have a uniform film thickness. More specifically, higher performance of an electrophotographic photoreceptor can be realized by applying a thinner photosensitive layer with a uniform thickness. Therefore, new application methods have been under development and examination in order to allow application at lower cost.
As methods for forming a photosensitive layer by applying a coating composition for an electrophotographic photoreceptor onto a conductive substrate serving as a bare tube for a photoreceptor, for example, dip coating, roll coating, blade coating, and spray coating are conventionally known. However, these methods have problems such as inability of providing uniform coating films or poor production efficiency.
The dip coating method is often used in manufacturing an electrophotographic photoreceptor, in which method a conductive substrate serving as a bare tube for a photoreceptor is dipped while one end thereof being held and the surface subjected to application of the coating composition for an electrophotographic photoreceptor being held perpendicular to the liquid surface of the coating composition for an electrophotographic photoreceptor, and then lifted from the coating composition However, the film thickness of the layer that is obtained by the dip coating method depends significantly on the lifting speed at which the conductive substrate is lifted from the coating composition, the viscosity of the coating composition, and the evaporation speed of a volatile component contained in the coating composition. Therefore, these factors have to be controlled strictly. Furthermore, since the conductive substrate is lifted from the coating composition in the vertical direction, the coating composition flows down along the surface of the conductive support by gravity effect, and the film thickness of the conductive support on the lower side in the lifting direction is larger than that on the upper side. Thus, a difference in the sensitivity appears in the vertical direction. A coating composition for an electrophotographic photoreceptor applied onto a lower portion is difficult to dry, and when a next coating composition for an electrophotographic photoreceptor is applied before the previous one has been dried completely, these coating compositions for an electrophotographic photoreceptor are mixed, and thus a desired photosensitive layer may not be formed. Furthermore, a coating composition for an electrophotographic photoreceptor flows into an end portion that is to be a non-printed portion, and thus masking and a process of eliminating the coating composition for an electrophotographic photoreceptor are necessary. In addition, in order to dip a conductive substrate, a coating composition for an electrophotographic photoreceptor in at least an amount that is sufficient for dipping the conductive substrate is necessary, and there is no choice but to dispose of the coating composition for an electrophotographic photoreceptor after a period during which the coating composition for an electrophotographic photoreceptor can be used (pot life), and thus the use efficiency of the coating composition for an electrophotographic photoreceptor is poor.
In roll coating, a film of a coating composition for an electrophotographic photoreceptor with a regulated film thickness is formed on a coating roll, and while a conductive substrate located near or in contact with the coating roll and the coating roll are rotated, the coating composition for an electrophotographic photoreceptor is transferred and applied from the coating roll onto the conductive substrate. However, when detaching the coating roll and the conductive substrate after the application, a so-called liquid trailing phenomenon, that is, a phenomenon in which an extra coating composition for an electrophotographic photoreceptor is attached to the conductive substrate by the surface tension of the coating composition for an electrophotographic photoreceptor tends to occur, and a joint remains in the coating film because of this liquid trailing phenomenon, and the film thickness becomes non-uniform. As a result, defects occur in the images.
In the blade coating method, a blade is located near a conductive substrate, a coating composition for an electrophotographic photoreceptor is supplied to the blade, the coating composition for an electrophotographic photoreceptor is applied onto the conductive substrate with the blade, and the blade is withdrawn after one rotation of the conductive substrate. The method provides high productivity, but when the blade is withdrawn, a part of the coating film applied to the conductive substrate is protruded by surface tension of the coating composition and the film thickness becomes non-uniform.
In the spraying coating method, coating is performed by spraying a coating composition for an electrophotographic photoreceptor from a spray nozzle in the form of fine particles, and therefore the appearance after coating is good. However, since a thickness of a layer formed by one coating is small, it is necessary to repeat coating a plurality of times in order to obtain a desired thickness. Furthermore, when a large amount of coating composition for an electrophotographic photoreceptor is applied for coating, the coating composition for an electrophotographic photoreceptor is dropped, and a coating layer with non-uniform thickness is formed. Furthermore, a coating composition for an electrophotographic photoreceptor is ejected from spray nozzles in the form of a cone, and therefore the precision of application is poor and masking is necessary in order to prevent, for example, the coating compositions for an electrophotographic photoreceptor from flowing into an end portion of a conductive substrate. In addition, the application efficiency is poor, so that an apparatus for collecting a coating composition for an electrophotographic photoreceptor that is not applied is necessary, and a processing apparatus is newly necessary in order to reuse the collected coating composition for an electrophotographic photoreceptor. Thus, processes other than application increase, and the production efficiency becomes poor.
As another method than the above, a method in which a coating composition for an electrophotographic photoreceptor is applied by ink-jet coating is used. In the method in which a coating composition for an electrophotographic photoreceptor is applied by ink-jet coating, while relatively moving an object to be coated and ejection nozzles, a coating composition for an electrophotographic photoreceptor is ejected from the minute ejection nozzles in the form of droplets and is attached to the object to be coated. Examples of systems for ejecting a coating composition for an electrophotographic photoreceptor from the ejection nozzles include piezoelectric ejection in which a coating composition for an electrophotographic photoreceptor is pressed out and ejected by a vibration of piezoelectric elements (piezo elements), Bubble Jet (registered trademark) in which voltage is applied to a heater, bubbles are generated in a coating composition for an electrophotographic photoreceptor, so that the coating composition for an electrophotographic photoreceptor is ejected, and thermal ink-jet ejection in which voltage is applied to a heater, bubbles are generated in a coating composition for an electrophotographic photoreceptor, the bubbles are collapsed, so that the coating composition for an electrophotographic photoreceptor is ejected.
A typical conventional technique is described in JP H11-19554A. In a method for applying a coating solution (a coating composition for an electrophotographic photoreceptor) disclosed in JP H11-19554A, a coating solution is applied by ink-jet coating.
As another conventional technique, a technique similar to that in JP H11-19554A is described in Japanese Patent No. 2644582. In a method for producing an electrophotographic photoreceptor disclosed in Japanese Patent No. 2644582, coating is performed by applying a pressure to let a coating material for forming an electrophotographic photoreceptor fly in the form of stripes from a plurality of minute openings continuously.
In ink-jet coating, a coating composition for an electrophotographic photoreceptor is ejected in the form of droplets from the minute ejection nozzles each having a size of several tens μm to form a charge-generating layer. Therefore, it is important to perform ejection in a stable manner without causing clogging due to dryness or aggregation of a pigment. Generally, in a coating composition for an electrophotographic photoreceptor having a high viscosity, sedimentation of a pigment tends not to occur and the uniformity can be ensured easily, but ejection performance becomes poor. In a coating composition for an electrophotographic photoreceptor having a low viscosity, ejection performance can be ensured easily, but sedimentation and aggregation of a pigment tend to occur, which may cause clogging of the nozzles.
In the method for applying a coating solution disclosed in JP H11-19554A, the method uses ink-jet coating, and thus ejected droplets can fly in the form of lines with an extremely high precision, and each of nozzles can be controlled, so that masking is not necessary and the application efficiency is extremely high. Furthermore, the coating solution can be replaced easily only by replacing a tank for storing the coating solution, and the coating composition for an electrophotographic photoreceptor can be used up, so that the production efficiency is extremely high. However, in the coating solution, tetrahydrofuran is used as a solvent. The boiling point of tetrahydrofuran is so low that the solvent volatilizes soon and thus the coating solution is dried in the ejection nozzles, and thus the nozzles are clogged. Furthermore, the uniformity of the film thickness after application (leveling performance) is not sufficient.
In the method for producing an electrophotographic photoreceptor disclosed in Japanese Patent No. 2644582, a pressure is applied to let a coating material for forming an electrophotographic photoreceptor be applied from a plurality of minute openings, and thus the application can be performed onto a large area unlike the application method using ink-jet coating. However, in this method, each of the minute openings cannot be controlled one by one, and there is a time lag between pressure application and ejection since the pump for applying a pressure and the minute openings are connected by a tube, so that both of the precision and the response become poor.